1. Field of the Invention
The present invention relates to an inertia calculating method and an electric motor driver. More particularly, it relates to the inertia moment (inertia) calculating method and the electric motor driver at the time of executing the velocity control of an inductive electric motor.
2. Description of the Related Art
When executing the velocity control of an electric motor, the mechanical inertia becomes necessary as a control constant. As a prior art for measuring the inertia, JP-A-61-88780 has disclosed the following method: The acceleration and the deceleration are executed at the velocity-changing rates the absolute values of which are the same in the same velocity differences (their velocity width xcex94xcfx89r). Then, the acceleration torque xcfx84ac and the deceleration torque xcfx84d are calculated from the respective torque proportion signals so as to calculate the inertia J from the integrated quantities of the respective torques during the acceleration and the deceleration. Hereinafter, the method will be explained in detail:
FIG. 4 is a diagram for illustrating the motor torque xcfx84m, the load torque xcfx84L, and the acceleration torque xcfx84ac in the case of performing the calculation of the inertia J in accordance with the prior art. In FIG. 4, letting the motor machine""s angular velocity be abbreviated as xcfx89, the relation holding between J and the torques is given by the equation (1):
                              J          ⁢                                    ⅆ              ω                                      ⅆ              t                                      =                                            τ              m                        -                          τ              L                                =                      τ                                          a                ⁢                                  xe2x80x83                                ⁢                c                            ⁢                              xe2x80x83                                                                        (        1        )            
The velocity difference xcex94xcfx89 caused by the acceleration at the acceleration time-period is equal to the velocity difference xcex94xcfx89 caused by the deceleration at the deceleration time-period. Integrating both sides of the equation (1) to determine J from the torques at the acceleration and the deceleration time-periods, J is given by the equation (2):
                    J        =                                                            ∫                ta2                ta1                            ⁢                                                (                                                            τ                      m                                        -                                          τ                      L                                                        )                                ⁢                                  ⅆ                  t                                                      Δω                    =                                                    ∫                ta3                ta4                            ⁢                                                (                                                            τ                      m                                        -                                          τ                      L                                                        )                                ⁢                                  ⅆ                  t                                                                    -              Δω                                                          (        2        )            
Determining once again J by averaging the above-described J values calculated at the acceleration and the deceleration time-periods, J is represented by the equation (3):
                    J        =                              1            2                    ⁢                      {                                                                                ∫                    ta2                    ta1                                    ⁢                                                            (                                                                        τ                          m                                                -                                                  τ                          L                                                                    )                                        ⁢                                          ⅆ                      t                                                                      Δω                            +                                                                    ∫                    ta3                    ta4                                    ⁢                                                            (                                                                        τ                          m                                                -                                                  τ                          L                                                                    )                                        ⁢                                          ⅆ                      t                                                                                        -                  Δω                                                      }                                              (        3        )            
Here, since the acceleration and the deceleration are executed in the same velocity differences during the same time-periods, the integrated values of the load torque xcfx84L during the acceleration and the deceleration time-periods become equal to each other:                                           ∫            ta2            ta1                    ⁢                                    τ              L                        ⁢                          ⅆ              t                                      =                              ∫            ta3            ta4                    ⁢                                    τ              L                        ⁢                          ⅆ              t                                                          (        4        )            
Accordingly, from the equations (3) and (4), J is determined from xcfx84m alone as is expressed by the equation (5):                     J        =                                                            ∫                ta2                ta1                            ⁢                                                τ                  m                                ⁢                                  ⅆ                  t                                                      -                                          ∫                ta3                ta4                            ⁢                                                τ                  m                                ⁢                                  ⅆ                  t                                                                          2            ⁢            Δω                                              (        5        )            
Using a detected torque current IqFB, the value of xcfx84m can be calculated as is expressed by, e.g., the equation (6):                               τ          m                =                              3            ⁢                          (                              P                2                            )                        ⁢                                          M                                  L                  2                                            ·                              MI                d                *                            ·                              I                qFB                                              ≡                                    Δ              0                        ·                          I              qFB                                                          (        6        )            
where, P, M, L2, and Id* denotes the following, respectively: The motor pole number, the motor mutual inductance, summation of the motor mutual inductance and the motor secondary-side leakage inductance, and the magnetic field excitation current instruction. Based on the above-described explanation, J is calculated from the equations (5) and (6).
In this method, the cancellation of the load torques xcfx84L makes it possible to calculate the inertia J independently of the form of the load torque.
In the method disclosed in JP-A-61-88780, however, as will be pointed out below, the motor is in a danger of being transitioned into a regenerative state at the deceleration time-period. This regenerative state overcharges, e.g., a smoothing capacitor within an inverter, thereby damaging the capacitor.
In FIG. 4, the motor torque xcfx84m becomes the lowest at the deceleration-terminating time (t=ta4). At this time, the torque current Iq also becomes its minimum. Assuming that the load torque xcfx84L is proportional to the square of the angular velocity xcfx89 (i.e., square load), Iq is determined from the equations (1) and (6) as is expressed by the equation (7): Incidentally, the reference notations therein denote the following, respectively: xcfx89 the motor velocity, xcfx890 the rated motor velocity, dxcfx89/dt the velocity-changing rates (the acceleration and the deceleration rates), P, the motor pole number, M, the motor mutual inductance, L2 the summation of the motor secondary-side leakage inductance and M, Id* the magnetic field excitation current instruction, J the mechanical inertia, and, Iq0 the rated motor torque current.                                                                         I                q                            =                              xe2x80x83                            ⁢                                                                                          (                                              ω                                                  ω                          0                                                                    )                                        2                                    ·                                      I                    q0                                                  +                                                      1                                          3                      ⁢                                              (                                                  P                          2                                                )                                            ⁢                                                                        M                                                      L                            2                                                                          ·                                                  MI                          d                          *                                                                                                      ⁢                                                            ⅆ                      ω                                                              ⅆ                      t                                                        ⁢                  J                                                                                                        =                              xe2x80x83                            ⁢                                                                                          (                                              ω                                                  ω                          0                                                                    )                                        2                                    ·                                      I                    q0                                                  +                                                      1                                          Δ                      0                                                        ⁢                                                            ⅆ                      ω                                                              ⅆ                      t                                                        ⁢                  J                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  Δ                        0                                            =                                              3                        ⁢                                                  (                                                      P                            2                                                    )                                                ⁢                                                                              M                                                          L                              2                                                                                ·                                                      MI                            d                            *                                                                                                                )                                                                                                          (        7        )            
As a result, there exist some cases where the minimum value of Iq (i.e., the equation (7)) becomes negative and thus the motor is transitioned into the regenerative state, because the deceleration rate dxcfx89/dt is negative at the deceleration time-period. As seen from the equation (7), the condition under which the minimum value of Iq becomes negative and the motor is transitioned into the regenerative state is the case where the deceleration is executed in a region of small xcfx89 (the load torque) and |dxcfx89/dt|, i.e., the deceleration rate at that time, is large. consequently, in order to prevent the regenerative state from occurring at the deceleration time-period, it becomes absolutely required to reduce the deceleration rate (=the acceleration rate). In that occasion, however, the acceleration or the deceleration torque does not become larger enough as compared with the motor torque and the load torque components that become an error. This gives rise to an expectation that the inertia-identifying accuracy will become worse.
It is an object of the present invention to provide an inertia calculating method and an electric motor driver that are preferable for calculating the inertia and for driving an electric motor without causing the regeneration to occur and based on a configuration that is simpler as compared with the configuration in the prior art.
In order to accomplish the above-described object, in a driver including a non-regenerative type power converter and executing the velocity control of the electric motor with the use of a mechanical inertia constant, the non-regenerative type power converter being a converting apparatus for converting an alternating current from an alternating power supply into an alternating current of a variable voltage and a variable frequency, the non-regenerative type power converter including a forward converter for converting the alternating current from the alternating power supply into a direct current, a smoothing capacitor connected to a direct current circuit, and a backward converter for converting the direct current into the alternating current, when calculating the mechanical inertia, the mechanical inertia is calculated during only the motor acceleration time-period so that a voltage of the smoothing capacitor included in the non-regenerative type power converter will not exceed a predetermined value.
Also, when calculating the mechanical inertia, the accelerations are executed at a plurality of times at the mutually different velocity-changing rates, and the mechanical inertia is calculated from the integrated quantities of the respective torque proportion signals and the velocity-changing widths.
Also, in a driver including a power converter and executing the velocity control of the electric motor with the use of a mechanical inertia constant, the power converter including a forward converter for converting an alternating current from an alternating power supply into a direct current, a smoothing capacitor connected to a direct current circuit, and a backward converter for converting the direct current into an alternating current, the power converter converting the alternating current from the alternating power supply into the alternating current of a variable voltage and a variable frequency, when calculating the mechanical inertia from the integrated quantities of the torque proportion signals and the velocity-changing widths at the time of changing the rotation velocity of the electric motor, the accelerations are executed at a plurality of times at the mutually different velocity-changing rates, and the mechanical inertia is calculated from the integrated quantities of the respective torque proportion signals and the velocity-changing widths.
According to the present invention, in comparison with the prior art method, the mechanical inertia is calculated during only the motor acceleration time-period. This condition allows the identification of the inertia J to be executed without causing the regeneration to occur.
Also, according to the present invention, the accelerations are executed at the plurality of times at the mutually different velocity-changing rates, thereby calculating the mechanical inertia J. This condition makes unnecessary the data at the deceleration time-period, which has been required in the prior art method. As a result, it becomes possible to apply the present invention to a regeneration operation-incapable inverter as well. Also, in particular, it becomes possible to simplify the configuration of an electric motor driver including the regeneration operation-incapable inverter.